The prior art relates various foams which can be employed in absorbency applications. Two varieties are high internal phase emulsion (HIPE) foams and extruded, open-cell thermoplastic foams. HIPE foams are seen by example in U.S. Pat. Nos. 5,372,766 and 5,387,207 and extruded, open-cell thermoplastic foams are seen by example in Canadian Patent Application 2,129,278 and Japan Application No. 2-120339.
HIPE foams are formed by the cross-linking polymerization of hydrophobic monomers as the continuous phase of a water-in-oil emulsion in which the water phase comprises at least 70 weight percent and typically greater than 95 weight percent. The structure of HIPE foams depends on their composition and process for making, but the most desirable ones for absorbing large amounts of fluid are substantially open-cell with thin cell walls containing numerous pores therein in communication with neighboring cells. HIPE foams can be prepared which exhibit relatively high absorption rates and have absorption capacities of greater than 25 grams of water per gram of foam. Thus, HIPE foams are very useful in absorbing fluids. HIPE foams are costly, however, due to the large volumes of water used in their preparation.
Extruded, open-cell thermoplastic foams typically have substantially more internal structure than HIPE foams. They typically are formed of interconnecting struts and walls with the open cell character being derived from a relatively small number of small diameter pores within relatively thick cell walls. Struts are formed by the intersection of cell walls. The relatively substantial internal cell structure and small pores in the cell walls induce viscous drag and resistance to flow within the foam. The relatively thick cell walls reduce the amount of fluid that can be absorbed within the foam. The relatively small number of small diameter pores may result in some portions of the foam not being accessible to the absorption of fluid. Thus, prior art extruded, open-cell foams, even those of essentially 100 percent open-cell content, typically exhibit both relatively low absorption capacity and a relatively low slow absorption rate.
It would be desirable to have an extruded, open-cell thermoplastic foam which exhibited both high absorption capacity and high absorption rate. It would also be desirable if absorption rate could be enhanced in specific directions or dimensions within the foam.